A waveguide is an apparatus for transmitting an electromagnetic wave in radio fields such as radio communication, radars, and navigation, and is a basic circuit unit in a circuit system. Generally, a circuit system has multiple waveguides, and therefore, adaptation is required between a waveguide and another waveguide or between a waveguide and another sub-circuit. However, if a filter with a frequency-selecting function, that is, a waveguide filter, is formed in an adaptation process, a quantity of filters in the circuit system may be reduced to some extent.
A waveguide filter commonly used in a microwave and millimeter wave circuit may be filter based on a metal waveguide and a filter based on a planar circuit such as a microstrip line and a coplanar line. The filter based on a metal waveguide generally has advantages such as a high Q value (quality factor), a low loss, and desirable selectivity. The filter based on planar circuit technologies such as a microstrip line and a coplanar line has a feature of easy integration into an active circuit. A filter based on a substrate integrated waveguide has such advantages of a planar circuit as being easily integrated and conveniently manufactured, and also has excellent performance similar to that of a metal waveguide filter.
However, the foregoing waveguides that form a filter are generally disposed at a same layer of circuit. When the waveguides are applied to multiple layers of circuits, an extra transition structure needs to be added to implement inter-layer adaptation, which imperceptibly increases complexity of a circuit structure and a circuit loss.